#4237 implicit_plot punting to plot2d implicit fails when argument is a list of equations

[Robert Dodier]

Since commit [ 14b3834f ] , implicit_plot (share/contrib/implicit_plot.lisp) has punted to plot2d, rewriting the first argument (call it foo) as foo = 0. However, foo might be a list of equations, and in that case, foo = 0 is incorrect; instead, implicit_plot should look at each element of the list and rewrite them one by one.

Discovered by this Stackoverflow question: https://stackoverflow.com/questions/77738791/plot2d-nothing-to-plot-for-error-when-attempting-to-plot-with-wximplicit-plot

[Robert Dodier]

Fixed by commit [ 65bc099 ].